1. Field of the Invention
The present invention relates to a voltage regulator including an overcurrent protection circuit.
2. Description of the Related Art
A conventional voltage regulator is described. FIG. 6 is a circuit diagram illustrating the conventional voltage regulator.
A differential amplifier circuit 104 compares an output voltage of a reference voltage circuit 103 and an output voltage of a voltage dividing circuit 106, and maintains the output terminal voltages of the reference voltage circuit 103 and the voltage dividing circuit 106 at the same level to control a gate voltage of an output transistor 105 so that a voltage of an output terminal 102 is kept at a predetermined voltage.
Here, if the output voltage of the voltage regulator decreases because of an increased load, an output current Iout increases up to a maximum output current Im. Then, in accordance with the maximum output current Im, a large amount of current flows through a sense transistor 121 that is current-mirror-connected to the output transistor 105. On this occasion, a P-channel transistor 601 is turned ON to increase a voltage generated by a single resistor 602, and an N-channel enhancement type transistor 124 approaches an ON state to increase a voltage generated by a resistor 122. Then, a P-channel transistor 125 approaches an ON state to decrease a gate-source voltage of the output transistor 105, with the result that the output transistor 105 approaches an OFF state. Consequently, not exceeding the maximum output current Im, the output current Iout is fixed to the maximum output current Im to decrease an output voltage Vout. Here, the output current Iout is fixed to the maximum output current Im when the gate-source voltage of the output transistor 105 decreases based on the voltage generated by the single resistor 602 and the output transistor 105 approaches the OFF state. Therefore, the maximum output current Im is determined by a resistance of the resistor 602 and a threshold voltage of the N-channel enhancement type transistor 124.
When the output voltage Vout decreases and then a gate-source voltage of the P-channel transistor 601 decreases to be lower than an absolute value Vtp of a threshold voltage of the P-channel transistor 601, the P-channel transistor 601 is turned OFF. A voltage is then generated by both the resistor 602 and a resistor 603, not the single resistor 602, which is so high that the N-channel enhancement type transistor 124 further approaches the ON state. Accordingly, the voltage generated by the resistor 122 further increases and the P-channel transistor 125 further approaches the ON state, with the result that the gate-source voltage of the output transistor 105 further decreases and the output transistor 105 further approaches the OFF state. Consequently, the output current Tout reduces up to a short-circuit current Is. The output voltage Vout thereafter decreases to 0 V. Here, the output current Tout reduces to the short-circuit current Is when the gate-source voltage of the output transistor 105 decreases based on the voltage generated by both the resistors 602 and 603 and the output transistor 105 approaches the OFF state. Therefore, the short-circuit current Is is determined by resistances of both the resistors 602 and 603 (see, for example, Japanese Patent Application Laid-open No. 2003-216252 (FIG. 5)).
In the conventional technology, the maximum output current Im and the short-circuit current Is are determined by the resistances of both the resistors 602 and 603 and the threshold voltage of the N-channel enhancement type transistor 124. Therefore, for accurate setting of the maximum output current Im and the short-circuit current Is, a trimming process is required to set the resistances of the resistors 602 and 603 accurately, which is a problem of the conventional technology that a manufacturing process is complicated.